The recovery of copper and/or precious metals is routinely conducted by hydrometallurgical processes. Different types of reagents have been used to leach copper and/or precious metals. Many of those reagents have disadvantageous properties, such as toxicity, expense, lack of selectivity and low extraction rates, as is discussed in detail below.
Precious Metals
In the leaching of precious metals such as gold and/or silver, lixiviants include cyanide, thiosulfate, thiocyanate, halides and halogens (such as chlorides, bromides and iodides and chlorine, bromine and iodine) and thiourea. Of these, cyanide remains the predominant reagent that is applied on an industrial scale for gold and gold-silver ores, although copper ammoniacal thiosulphate systems have been implemented at some gold mining and processing sites at an industrial scale as of 2012. Halides (chlorine and chlorides in particular) are often used in the final refining of impure bullion and doré (gold-silver alloy). In addition, highly poisonous and environmentally hazardous metallic mercury is often still used by many artisanal miners. Despite being a robust leaching reagent (lixiviant), sodium cyanide, and cyanides of other alkali (such as potassium) metals and alkali earth metals (such as calcium), all referred to as “Cyanide”, use poses a number of challenges, principally due to its toxicity, regulatory restrictions, high carbon footprint and low selectivity in low grade ores. It is particularly problematic for gold ores with high copper and/or high silver content as copper is often present at levels of around 1000 times the gold concentration (silver often 5-50 times gold concentration), leading to excessive cyanide consumption, and removal of available cyanide for gold leaching. Cyanide is also an expensive reagent so that using it for lower value metals such as copper (and less so, for silver) quickly becomes uneconomic, not only for leaching, but also due to downstream impacts (eg, competition with gold during adsorption onto activated carbon, elution and waste treatment). In addition it generates weak acid dissociable (WAD) cyanides which require cyanide detoxification/destruction or recovery processes.
The current alternative lixiviants to cyanide also pose many challenges. Despite sodium thiosulphate being the main contender as a lixiviant for gold, it is expensive, it requires additional copper (as Cu2+) as an oxidant (if not already present in the gold ore) and volatile and noxious ammonia to stabilise the leaching system. It is applicable to only a limited number of gold ores. Further, it cannot economically be produced at site, it requires complex downstream separation and it is not biodegradable.
The key challenges of alternative inorganic lixiviants are set out below:
LixiviantDetractorsSodium thiocyanate inPoor solubility of silver often associated withacid mediagold.Corrosion due to acid environmentToxicityVery stable (poorly biodegradable)Cannot be produced on siteThioureaCarcinogen and toxic.ExpensiveDangerous to the environmentCannot be produced on site.Halides (chloride-Limited to gold in flotation concentrate ratherbromide-iodide)than low grade ore.Corrosion and high capital costsPoor selectivity (mobilisation of most metals).Poor solubility of other precious metals suchas silver.Not biodegradable.Damaging to the environment.Copper
Currently, more than 20% of world copper production is produced by using hydrometallurgical processes, particularly by acidic heap leach (or heap bioleach)-solvent extraction-electrowinning processes for low grade ores.
The presence of copper minerals with gold is known to lead to many challenges during the cyanidation of gold ores, such as high consumption of cyanide with low gold extraction and undesirable impacts on gold recovery during the downstream processes. One previous process for the treatment of such ores has focussed on selective leaching of gold from copper-gold ores with ammonia-cyanide. However, the success of this process is sensitive to the type of ore. While this process can be effective for treating oxidised ores, it was found that the treatment of transition or sulphidic ores gave poor gold recovery and required high reagent concentrations. Copper minerals consume about 30 kg/t NaCN for every 1% of reactive copper present, making conventional cyanidation of copper-gold ores or concentrates uneconomic. Moreover, both cyanide and ammonia have detrimental environmental effects.
Of the copper sulphide minerals, chalcopyrite is the most refractory to leaching in acidic media (sulphuric acid, on its own or in conjunction with hydrochloric or nitric acids, or associated with bioleaching) which is the conventional hydrometallurgical process to extract copper, and poor copper recovery is the norm with elevated temperatures being required for acceptable copper extraction. Various passivation layers have been noted to form depending on the leach conditions, which slows the leach reaction significantly, or stops the reaction from proceeding, depending on the surface nature of chalcopyrite and the particular chemical leach conditions used. In gold-copper ores, slow leaching chalcopyrite contributes to cyanide consumption and, in some cases, occlusion of gold.
There are a number of large copper-gold mines in Australia and the Asia-Pacific region. These plants produce copper-gold concentrates and ship them for copper smelting and gold recovery from anode slimes (e.g. Telfer, Mt Carlton, Boddington and Cadia Valley, etc.). However, the increase of gold bearing-pyrite and gold bearing-arsenian pyrite content in the copper ores results in the production of low grade copper concentrate. In addition, the presence of arsenic limits of the flotation mass pull of chalcopyrite rich concentrates, so that a significant portion of the gold has to be recovered by gravity and leaching of the flotation tails. The transportation of the low grade concentrate overseas is often uneconomical. Therefore, an alternative process would be useful for recovering copper and/or gold from such a low-grade or difficult ores.
There is accordingly a need for an alternative metal recovery process that uses cheap and environmentally benign lixiviants for precious metal and/or copper recovery. There is also a need for an environmentally friendly recovery process with low (operating and capital) cost as an alternative extraction method for copper and/or precious metals. There is also a need for an environmentally friendly recovery process can be used to leach copper and/or gold and/or silver through in-situ leaching, in place leaching, heap leaching or vat leaching of either ore or mineral concentrates. There is a further need for an efficient and environmentally friendly process for selectively recovering copper from copper/precious metal ores and ore concentrates.